Fender Blues Deville 1994 - Cutting Out

[MDF99]

After the amp is warmed up and when I'm playing hard or turn on the tube driver pedal while playing hard this amp will cut out, no sound. When it cuts out while I'm in the clean channel, the red light for the drive channel will come on and burn dimly. If it cuts out while I'm in the drive channel then bright drive light will go dim.

While the amp is in this cut-out state with no sound, I can press on either one of the 330 Ohm 5 Watt ceramic screen resistors with my guitar pick and the amp will work until I release pressure.

So it seems there's a bad solder joint that breaks contact when things are heated up.

Can anyone explain what these resistors are doing and how I can perform a repair? When the signal from my guitar and pedal is strong does it cause these resistors to get hot? I don't know what a screen resistor is supposed to do. Are there any other parts in this amp a novice like me should check out?

The preamp tubes are GT ECC83, power tubes are 6L6GC.

Thanks.

[Enzo]

You have discovered the problem. Those 5w resistors are marginal there, and they get hot enough to unsolder themselves.

Personally I might go back to 470 ohm resistors. SOme guys get little Dale or similar brand aluminum power resistors, mount them on the chassis, and run wires. Those woul;d be more like 25 watt.

The resistors are dropping resistors for the 15v zener supplies your low voltage circuits run from.

[Mark Black]

Hi MDF99 -

Good job narrowing down the source of the problem.

Those resistors are current-limiters for zener diodes that create the +/- 16VDC section of the power supply, which is used for the channel switching as well as other solid-state portions of the amp. They are known to run hot and cook off the solder at their connection points, and have been the topic of much previous discussion on this forum. The heat they dissipate is not affected by the guitar signal in any way. They are NOT screen resistors.

A "quick & dirty" repair would be to suck off the old solder, clean the resistor leads and then re-solder the resistors.

However getting to the solder-side of the PCB involves a fair amount of work, so if this isn't an emergency a better solution might be to replace the resistors. New ones will give you some longer leads to play with so you can scrape a bit of solder-mask off the PCB traces and bend those leads over for about a 1/4 inch or so to give a much larger solderable area for heat dissipation.

Also, after doing the work to get the boards out, this would be a perfect time to retouch (and that means adding solder - not just re-melting) the solder joints at:

- ALL of the jack solder points.
- ALL of the pot solder points (including the mounting bracket pins).
- ALL of the switch solder points.
- ALL of the LED solder points.
- ALL of the large capacitor solder points.
- ALL of the wire plug-on quick-disconnect terminals.
- ALL of the tube socket pins.
The large zener diode solder points (these may need to have their leads cleaned to take solder well if they have been running hot, and they almost certainly have).
And any other joint you see along the way that looks like it might be cracked or lacking sufficient solder.

Try not to bend the wiring between the two PC boards more than an absolute minimum - the conductors tend to break where they enter the PC boards and every time they are flexed they get a little closer to being broken. This is another reason to get all the solder work done while the boards are out for the resistor fix.

Other than that I can't think of anything else to go after offhand.

Of course all of the above assumes (hopes for?) a certain amount of soldering skill & mechanical aptitude on your part, as well as a good soldering iron & the correct solder.

Hope things go smoothly and best-of-luck!

Edit: see - I typed too long and Enzo beat me to it... :-)

[Enzo]

AH, you would have beaten me, I just didn't run as long as usual. I must be off my feed.

[drewl]

What Mark described is pretty much the standard routine I have for servicing the Hot rod series.
Had a Deville and Deluxe this week.

[MDF99]

Thanks for the help and advice guys. I'll open up this amp and replace the 330 with 470 ohm resistors and re-solder all the connections.

[ekulas]

I also am having a problem with the channel switching and in my search for a solution I have found that I need to replace the resistors. I went to an electronics store and was about to buy some resistors and I ran into the following problems: Is it OK to have a different value of 5W resistors ie. the stock resistors are 330 ohm and I'm wondering if it will mess it up to replace it with 470 ohm. Also, the retailers offered me 10W resistors with a value of 330 ohm, would that make a difference?

[52 Bill]

The recommended value for these resistors is 470 ohms 5 watt. If yours has the 390 ohm values replace them with the 470 ohm ones.

You can use 10 watt resistors but there will be no room to mount them on the pc board. These resistors get hot and are large enough to need secure mounting.